Oxonium ion

In

chemistry

, an oxonium ion is any

oxygen

cation with three bonds. The simplest oxonium ion is the

hydronium

ion H₃O⁺.

Alkyloxonium

Hydronium is one of a series of oxonium ions with the formula R_''n''H_3−''n''O⁺.
Oxygen is usually pyramidal with an sp³ hybridization. Those with ''n'' = 1 are called primary oxonium ions, an example being protonated alcohol (e.g.

methanol

). In acidic media, the oxonium functional group produced by protonating an alcohol can be a leaving group in the

E2 elimination

reaction. The product is an

alkene

. Extreme acidity, heat, and dehydrating conditions are usually required. Other hydrocarbon oxonium ions are formed by protonation or

alkylation

of alcohols or

ether

s (R−C−−R₁R₂).

Secondary oxonium ions have the formula R₂OH⁺, an example being protonated ethers.

Tertiary oxonium ions have the formula R₃O⁺, an example being

trimethyloxonium

. Tertiary

alkyl

oxonium salts are useful alkylating agents. For example,

triethyloxonium tetrafluoroborate

()(), a white crystalline solid, can be used, for example, to produce ethyl esters when the conditions of traditional Fischer

esterification

are unsuitable. It is also used for preparation of enol ethers and related functional groups.

Oxatriquinane

and

oxatriquinacene

are unusually stable oxonium ions, first described in 2008. Oxatriquinane does not react with boiling water or with alcohols,

thiol

s, halide ions, or

amine

s, although it does react with stronger

nucleophile

s such as hydroxide,

cyanide

, and

azide

.

Oxocarbenium ions

Another class of oxonium ions encountered in organic chemistry is the

oxocarbenium

ions, obtained by protonation or

alkylation

of a

carbonyl

group e.g. R−C=−R′ which forms a resonance structure with the fully-fledged carbocation R−−O−R′ and is therefore especially stable:

:

Gold-stabilized species

An unusually stable oxonium species is the gold complex tris riphenylphosphinegold(I)xonium tetrafluoroborate, Ph₃PAu)₃OBF₄], where the intramolecular aurophilicity, aurophilic interactions between the gold atoms are believed responsible for the stabilisation of the cation. This complex is prepared by treatment of Ph₃PAuCl with

Ag₂O

in the presence of NaBF₄:

:3 Ph₃PAuCl + Ag₂O + NaBF₄ → Ph₃PAu)₃Osup>+ F₄sup>− + 2 AgCl + NaCl

It has been used as a catalyst for the propargyl Claisen rearrangement.

Relevance to natural product chemistry

Complex bicyclic and tricyclic oxonium ions have been proposed as key intermediates in the biosynthesis of a series of natural products by the red algae of the genus ''

Laurencia

''.

Several members of these elusive species have been prepared explicitly by total synthesis, demonstrating the possibility of their existence. The key to their successful generation was the use of a weakly coordinating anion (Krossing's anion, l(pftb)₄sup>−, pftb = perfluoro-''tert''-butoxy) as the counteranion. As shown in the example below, this was executed by a transannular halide abstraction strategy through the reaction of the oxonium ion precursor (an organic halide) with the silver salt of the Krossing's anion Ag l(pftb)₄��CH₂Cl₂, generating the desired oxonium ion with simultaneous precipitation of inorganic silver halides. The resulting oxonium ions were characterized comprehensively by nuclear magnetic resonance spectroscopy at low temperature (−78 °C) with support from density functional theory computation.

These oxonium ions were also demonstrated to directly give rise to multiple related natural products by reacting with various

nucleophile

s, such as water, bromide, chloride and acetate.

See also

*Onium ion, a +1 cation derived by protonation of a hydride (includes oxonium ions)
*

Pyrylium

, another type of ion with +1 formal charge on oxygen
*Sulfonium, a heavier congener

References

{{Reflist

Oxycations
Oxonium compounds